Link to publication in University of Groningen/UMCG research database . To determine the influence of water on the glass transition temperature of sugar glasses, the moisture content of humidified sugar glasses without protein/ ammediol. Our results provide insight into the relationship between relative humidity, moisture content and glass transition temperature for cell desiccation. the glass transition temperature (Tg) which is a moisture content dependent. .. for whey protein hydrolysates, no linear relationship between Tg and the inverse .
Glass transition - Wikipedia
In specific materials[ edit ] Silica, SiO2[ edit ] Silica the chemical compound SiO2 has a number of distinct crystalline forms in addition to the quartz structure. Nearly all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Si-O bond lengths vary between the different crystal forms.
Any deviations from these standard parameters constitute microstructural differences or variations that represent an approach to an amorphousvitreous or glassy solid. The transition temperature Tg in silicates is related to the energy required to break and re-form covalent bonds in an amorphous or random network lattice of covalent bonds.
The Tg is clearly influenced by the chemistry of the glass. For example, addition of elements such as BNaK or Ca to a silica glasswhich have a valency less than 4, helps in breaking up the network structure, thus reducing the Tg. Alternatively, Pwhich has a valency of 5, helps to reinforce an ordered lattice, and thus increases the Tg. For strong melts such as SiO2 the percolation threshold in the above equation is the universal Scher-Zallen critical density in the 3-D space e.
Polymers[ edit ] In polymers the glass transition temperature, Tg, is often expressed as the temperature at which the Gibbs free energy is such that the activation energy for the cooperative movement of 50 or so elements of the polymer is exceeded[ citation needed ].
This allows molecular chains to slide past each other when a force is applied. From this definition, we can see that the introduction of relatively stiff chemical groups such as benzene rings will interfere with the flowing process and hence increase Tg. When the glass temperature has been reached, the stiffness stays the same for a while, i.
This region is called the rubber plateau. In ironinga fabric is heated through this transition so that the polymer chains become mobile. The weight of the iron then imposes a preferred orientation. Tg can be significantly decreased by addition of plasticizers into the polymer matrix. Smaller molecules of plasticizer embed themselves between the polymer chains, increasing the spacing and free volume, and allowing them to move past one another even at lower temperatures.
The addition of nonreactive side groups to a polymer can also make the chains stand off from one another, reducing Tg. If a plastic with some desirable properties has a Tg that is too high, it can sometimes be combined with another in a copolymer or composite material with a Tg below the temperature of intended use. Note that some plastics are used at high temperatures, e.
The silicone toy Silly Putty behaves quite differently depending on the time rate of applying a force: On cooling, rubber undergoes a liquid-glass transition, which has also been called a rubber-glass transition.
Mechanics of vitrification[ edit ] Main article: Vitrification Molecular motion in condensed matter can be represented by a Fourier series whose physical interpretation consists of a superposition of longitudinal and transverse waves of atomic displacement with varying directions and wavelengths. In monatomic systems, these waves are called density fluctuations. These constants The analysis constitutes a sophisticated approach to were then used to predict water activity and moisture con- assess the viscoelastic nature of materials.
It provides read- tent for a given food material. The knowledge of sorption ings of the storage modulus G 0which is the elastic com- models of a given product is also useful in the modeling ponent of the network, loss modulus G00 ; viscous drying processes.
The water activity at monolayer moisture min. Thermal runs may be interrupted at constant temper- varied from 0.
Glass Transition and Moisture Content, Hydroxypropoxymethylcellulose Phthalate (HPMC-PH)
Glass transition temperatures that the small deformation analysis was carried out within the linear viscoelastic region. The samples having high moisture content show a freezing exotherm during cooling and the where Tgm, Tgs, and Tgw are the glass transition tempera- ture of the mixture, solids, and water respectively, Xs and Xw are the mass fraction of solids and water, and k is the Gordon—Taylor parameter.
From the thermodynamic 0. The model parameters k and 0. Comparison of two approaches In order to make a comparison between the two 0. The glass transition model Gordon—Taylor equation was then used to estimate the Tg at the corresponding sol- 0 ids content. Next, using the Gordon—Taylor equation, the 0 0. Experimental data for freeze dried abalone and predicted sorption aw was determined using the sorption isotherm model, i.
Based on the sorption isotherm criterion, for most food materials the predictions of the glass transition model Similarly, their study indicated that increasing molecular weight increased the Variation of glass transition temperature with solids for abalone. Roos pointed out that certain physicochemical and structural processes are better correlated to the glass glass transition event.
Hence annealing is necessary in transition temperature through plasticization by water or order to achieve maximal-freeze-concentration condition temperature. These include stickiness, crispness, collapse, and eliminate the exothermic peak if present.
It directly to Tg. In this context, Kasapis, Al-Marhoobi, was observed that the mechanical Tg could be pinpointed Deszczynski, Mitchell, and Abeysekera recently at the threshold of the free volume theory WLF and the demonstrated that addition of small amounts of gelling predictions of the reaction rate theory Arrhenius in the biopolymer to a high sugar preparation increases dramati- master curve of viscoelasticity.
The trough the conventional DSC Tg. The connection were made between the two concepts content was modeled using the Gordon and Taylor equa- and the moisture content of abalone samples for the tem- tion Fig.
This, of course, is too high for food preserva- tion and stability according to the postulates of the concept 3. Comparison of the concepts of water activity and of water activity, which requires a range of around 0.
The former argues that a product is ars i. The plasticizing activity value of about 0.
The latter suggests that formulations are stable at or below However, the level of plasticization by the same amount the corresponding glass transition temperature. The data S. In doing so, we should also requirements for product stability. Literature data on the derive a relationship between product characteristics e. These cesses perceive stability at a molecular level. A better understanding of S. CRC ling stability and food packaging requirement.
State diagram of tuna meat: Journal of References Food Engineering, 57, — Water activity and food del Valle, J. Food Research International, 31, — Rheological methods in the Fonseca, F. State diagrams characterisation of food biopolymers. Thermochimica Acta,— Food Technology, 34, 42— Water sorption, food stability. Journal of Food Processing and Preservation, 16, drying and antioxidant properties of dried tomato products.
Food Engineering, 52, — Applying state diagrams to food Gordon, M.
Ideal copolymers and the second processing and development. Food Technology, 45, 66— Glass transitions Journal of Applied Chemistry, 2, — Thermochimica Acta, Greenspan, L.
Physics and Chemistry, 81A 1 ,